This application claims benefits of priority under 35 U.S.C.119 to Japanese Patent Application No. P2000-89060 filed Mar. 28, 2000, the entire contents of which are incorporated by reference herein.
1. Field of the Invention
The present invention relates to a high frequency power amplifier using a bipolar transistor, more particularly to a high frequency power amplifier having high efficiency and low distortion, which uses a heterojunction bipolar transistor.
2. Description of the Background
For recent mobile telephones and mobile information terminals, transistors efficiently performing power amplification at a frequency band of 1 GHz or more have become indispensable constituent components. Among these transistors, a heterojunction bipolar transistor formed on a gallium arsenide (hereinafter referred to as GaAs) substrate is excellent in a high frequency characteristic and operates at a low voltage with high efficiency. Accordingly, the heterojunction bipolar transistor meets social demands for reducing the number of cells to lighten the telephones and the terminals, and attracts social attention. In addition, the heterojunction bipolar transistor shows less three-dimensional distortion, and has a characteristic suitable for digital modulation for which high linearity of operation is required.
Although the heterojunction bipolar transistor using a material of the GaAs group has the principally excellent characteristic, this transistor sometimes makes its characteristic deteriorated when it is intended to obtain large output power. This originates from the fact that heat conductivity of the GaAs substrate is as comparatively low as about 0.4 W/K/cm (about ⅓ of silicon), and a rise of the device temperature becomes large with an elevation of an output level. When the bipolar transistor is driven while keeping the base-emitter voltage thereof constant, it has been known that a collector current increases due to the temperature rise. Accordingly, a positive feedback of a current increase, a power increase, a temperature rise and a current increase is produced, in which the current increase creates the consumption power to rise the device temperature, thus further increasing the current. There is a drawback that unevenness of current distribution occurs in the high frequency power amplifier having a plurality of emitter fingers and a large area, and a thermal runaway state may be brought about at the worst, resulting in breakdown of the transistor.
To cope with such a problem, the most familiar method from way back is a ballast resistance method (G. Gao et al. IEEE Trans. Electronic Dev., 1991, pp. 185-196) for providing a ballast resistance which increases either an emitter resistance or a base resistance to apply a negative feedback to a current increase and a voltage relation between a base and an emitter, thus canceling a positive feedback due to a temperature rise. An example of a high frequency power amplifier by heterojunction bipolar transistors, which use the ballast resistance method, is shown in FIG. 1, and a high frequency power amplifier using the conventional bipolar transistors will be described.
In FIG. 1, an output voltage of a reference voltage generation circuit 12 for generating a reference voltage as a base bias is distributed to bipolar transistors 1a, 1b, 1c and 1d serving as fingers of a transistor circuit 10 via a bias generation circuit 2 for performing an impedance conversion by a transistor 11. The reference voltage as the base bias is adjusted in accordance with the temperature of a diode 6. The bias circuit having such constitution shall be called a diode bias circuit in the following descriptions.
An emitter of each transistor 1a, 1b, 1c and 1d is connected to an earthed electrode via corresponding one of ballast resistances 5a, 5b, 5c and 5d. A high frequency power is connected to a base of each of the transistors 1a, 1b, 1c and 1d of the transistor circuit 10 via a metal insulator metal (hereinafter referred to as MIM) capacitor device 4. To prevent the high frequency power from leaking to the base bias circuit, a resistance 3 is connected between an emitter of the impedance conversion transistor 11 and the high frequency power transistor 1. Accordingly the bias generation circuit 2 shown in FIG. 1 comprises a bipolar transistor 11 for impedance conversion, a resistance 3 for blocking a high frequency, and a resistance
FIG. 2 shows a pattern layout in a circuit constitution of the high frequency power amplifier using the conventional heterojunction bipolar transistor shown in FIG. 1. This pattern layout will be described in detail in the description of a first embodiment of the present invention while comparing with a pattern layout of a high frequency power amplifier as the first embodiment of the present invention. In the conventional power amplifier, 32 emitter fingers, each having a size of 4xc3x9730 xcexcm, are arranged in a chip of 1 mmxc3x972 mm as shown in FIG. 2, and a linear output of 30 dBmW is obtained. Here, reference numerals 1a, 1b, 1c and 1d denote transistors, each having eight emitter fingers connected in parallel. The bias circuit 2 composed of a diode bias circuit is arranged in the position shown in FIG. 2, and a DC potential is supplied to the base of each of the four transistor blocks 1a to 1d. The resistance 3 is provided for blocking the high frequency. A high frequency signal is connected to the base of each of the four transistor blocks 1a, 1b, 1c and 1d via the MIM capacitor 4.
So called a MMIC (Monolithic Microwave Integrated Circuit) is constituted by forming the transistor circuit having such constitution generally on a GaAs chip integratedly. In this circuit constitution, a change in temperature of the chip is detected by the diode 6, and a bias voltage in accordance with the temperature of the chip is supplied to the high frequency power transistor. However, when a high frequency power density becomes large, a temperature difference among the finger transistors of the high frequency power transistor circuit 10 occurs, thus making the current distribution uneven.
Particularly, temperature is apt to rise at the central portion of the high frequency transistor circuit 10, and in the example shown in FIG. 1, a sum of currents flowing in the finger transistors 1b and 1c is larger than that of currents flowing in the finger transistors 1a and 1d. In FIG. 3, the position of the transistor block in the conventional bipolar transistor circuit shown in FIG. 1 and the value of the collector current thereof are illustrated. As shown in FIG. 3, it is proved that the value of the collector current of the transistor positioned at the center of the bipolar transistor circuit varies more when the ballast resistance is 2 xcexa9 than when the ballast resistance is 3.5 xcexa9.
Generally, when the ballast resistances 5a to 5d are made to be larger, a resistance to thermal runaway increases, and uniformity of the current distribution can be improved. However, when the ballast resistances are made to be too large, a drawback occurs in which a voltage of the transistor at a saturated region increases, thus deteriorating efficiency and lowering a gain.
Even if the ballast resistances 5a to 5d are made to be larger in the conventional bipolar transistor circuit shown in FIG. 1 and the resistance to the thermal runaway of the high output transistor 10 can be increased, resistance to breakdown of the bias circuit may be a problem. This means a problems that when a large amount of the collector current flows through the transistor circuit 10 compared to a normal use because of fluctuation of an external additional resistance connected to the collector of the high output transistor circuit 10, the transistor 11 of the base bias circuit 2 is broken.
Specifically, when the collector current increases by fluctuation of an external load of the transistor circuit 10, the base current of the transistor circuit 10 also increases. The base currents of all of the transistors 1a, 1b, 1c and 1d of the transistor circuit 10 flow through the transistor 11 of the bias circuit 2. When the values of the base currents become too large, the transistor 11 makes thermal runaway so that the transistor 11 may be broken.
Also in the constitution shown in FIG. 4, the output potential of the base bias reference voltage generation circuit 12 is distributed to the bases of first bipolar transistors 1a, 1b, 1c and 1d as fingers, which perform high frequency power amplification via a bias circuit generation circuit 2 performing an impedance conversion and ballast resistances 7a, 7b, 7c and 7d. The reference voltage generation circuit 12 comprises a diode 6, and the bias generation circuit 2 comprises a second bipolar transistor 11 and a resistance 9 provided between the transistor 11 for impedance conversion and the ground.
In the circuit of FIG. 4 having the above-described constitution, a high frequency power is supplied to the bases of the finger transistors 1a, 1b, 1c and 1d via MIM capacitor devices 8a, 8b, 8c and 8d without passing through ballast resistances. In this method, though the values of the ballast resistances are made to be large to assure uniform operations of the transistors, since the high frequency power is directly input to the base terminals of the transistors, the drawback of the deteriorated efficiency of the high frequency power amplifier and the lowered gain thereof due to a voltage increase of the transistor at the saturated region does not occur, which has been the problem also in the high frequency power amplifier of FIG. 1, and the characteristic of the high frequency power amplifier is improved.
However, a problem occurs in the case of, for example, a CDMA (Code Division Multiple Access) modulation method in which signals such as modulation signals having envelopes largely billowing are dealt with. To be specific, if the ballast resistances are large, a frequency component of the envelope is superposed on a voltage applied between the intrinsic base and emitter of the heterojunction bipolar transistor, and cross modulation with a carrier frequency component occurs, thus deteriorating distortion.
The collector current of the bipolar transistor, to which the signal of the digital modulation method is input, is schematically shown in FIG. 5. In the digital modulation method, amplitude of a high frequency current also changes depending on time. In the circuit of the second conventional high frequency power amplifier shown in FIG. 4, though the high frequency current does not flow through the base ballast resistances 7a, 7b, 7c and 7d, an envelope component of the modulation signal that is a low frequency component flows through the ballast resistances 7a, 7b, 7c and 7d. Therefore, the voltage applied between the intrinsic base and emitter of the heterojunction bipolar transistor having a large ballast resistance is modulated by the frequency component of the envelope.
As a result, in the base terminals of the bipolar transistors 1a, 1b, 1c and 1d, the two signals such as the original modulation signal and the envelope signal cause cross modulation, thus deteriorating distortion. Accordingly, from the viewpoints of suppressing the distortion component, there has been a problem that the ballast resistance cannot be made to be large immoderately, and an amplifier that can be applied to the one for use in digital modulation, for which demands have recently increased, cannot be constituted.
To solve the foregoing problems, a first object of the present invention is to provide a bipolar transistor having an excellent uniformity in a current distribution in spite of a small ballast resistance, and capable of constituting an amplifier showing high efficiency and low distortion with little deterioration of distortion even when a digital modulation wave is input thereto.
In the conventional bipolar transistor shown in FIG. 4, when a large collector current flows due to load fluctuation of a collector of the transistor circuit 10, a current flowing through the transistor 11 of the bias circuit 2 becomes large, and the transistor 11 may be broken. This problem holds true for the first conventional bipolar transistor circuit shown in FIG. 1.
The object of the present invention is to provide a bipolar transistor excellent in uniformity of current distribution in spite of a small ballast resistance, and capable of constituting an amplifier showing high efficiency and low distortion with little deterioration of distortion even when a digital modulation wave is input thereto.
To achieve the foregoing object, the present invention has the following features.
A first high frequency power amplifier shown in the present invention of this application comprises a plurality of transistor blocks having a bipolar transistor, wherein each of said transistor blocks further includes: a resistance connected to an emitter of said bipolar transistor; a reference voltage generation circuit for generating a reference voltage as a base bias to be applied to a base of said bipolar transistor; and a bias generation circuit for generating a base bias voltage by converting said reference voltage, the bias generation circuit being connected to the base of said bipolar transistor.
According to a first structure of the present invention of this application, it is possible to supply the base bias potential in accordance with a change in temperature to each of the transistor blocks.
Furthermore, a second high frequency amplifier shown in the present invention of this application comprises: a plurality of transistor blocks having a bipolar transistor; and a reference voltage generation circuit for generating a reference voltage of a base bias for said bipolar transistor, wherein each of said transistor blocks further includes: a resistance connected to an emitter of said bipolar transistor; a bias generation circuit for generating the base bias by converting said reference voltage, the bias generation circuit being connected to the base of said bipolar transistor; and a capacitor device for high frequency input, the capacitor device being connected to the base of said bipolar transistor.
According to a second structure of the present invention of this application, it is possible to supply the base bias potential in accordance with a change in temperature to each of the transistor blocks.